Whales, seals, sea lions, catadromous fish such as eels and anadromous fish such as shad and salmon all migrate through confined waterways such as Puget Sound or Alaska's Inside Passage. The main means of determining populations and movement have been through spotting, i.e. human observers at chokepoints. Chokepoints have been traditionally at passages through which the migrating population must pass. Where the passages have been synthetic and narrow, such as fish ladders, or at natural constrictions, such as the mouths of streams, the movement of the migratory population has been readily estimated by visual sighting. Generally, the narrower the constriction, the better the count.
Only anadromous fish, however, migrate into narrow streams where such counts are facilitated by constrictions in the migratory waterway. Cetacean species rarely enter freshwater streams and will not readily be counted in such confined waterways. Yet the whales do pass through some well-defined passages on their migratory routes.
Orca whales, for example, can be observed to move between Puget Sound in Washington, through the Strait of Georgia in British Columbia, then on to southeastern Alaska; traveling through Prince William Sound to the waters around Kodiak Island. Given the breadth of any of several inland passages, visual counting lacks a great deal of the certainty necessary to accurately gauge the extent of the Orca population as they migrate.
Complicating the count is the fact that within the same waters, Humpback, Grey and other cetacean species live and move. Discernment of one whale species from another in the same space is also necessary for accurately assessing the population. All of these species regularly pass through the same defined chokepoints in the inland waterways. Because whale movement is predictable and includes swimming through predicted passages, accurate counts could be obtained by monitoring the seabed environment at each of these chokepoints to discern passage.
Selected sonar frequencies have been found to detect whales without imparting injury to the whale's own sonar guidance system. Sonar, however, only propagates in a cone defining a solid angle. Counting schemes to date have placed the sonar across the mouth of a chokepoint. Very few chokepoints, however, can be effectively monitored by positioning a single solid angle cone, for doing so requires the assumption that the whales will pass in single file through the chokepoints. Ganging sonar installations has been one strategy for enhancing the accuracy of counting results by placing several transducers along the migration path and comparing results to come up with counts that are in good agreement. A second ganging strategy consists of filing a chokepoint with several transducer cones, stacked to completely and non-overlappingly fill the chokepoint, but such ganging suffers from a lack of coordination of results such that a single whale might be counted more than once by passing through each of several sonar cones. Additionally, finding such chokepoint spaces that are readily filled with such transducer cones is difficult.
What is needed, then, is a predictable means of monitoring defined chokepoints with coordinated sonar installations. Comprehensive monitoring of chokepoints in defined seabed environments facilitates counting by allowing in-depth observation of all movement within those seabed environments.